Angiotensin-converting enzyme (ACE) is a clinically important enzyme whose elevated circulating levels are associated with the pathogenesis of essential hypertension, heart failure and renal failure. Consequently, inhibitors of this enzyme are widely used for clinical management of these diseases. ACE is a dipeptidyl carboxypeptidase whose natural substrates include the oligopeptides angiotensin I and bradykinin. It has to isozymes, ACEP and ACET, which have similar enzymic activities. The two isozymes of ACE are expressed in a tissue- specific manner: ACET expression is restricted to developing male germ cells, whereas ACEP is expressed in vascular endothelial cells, kidney tubular epithelial cells, intestinal brush border cells, monocytes and specific cell types in the brain. One of the major physiological functions of ACE that has been historically well-recognized, is its role in blood pressure regulation through its participation in the renin- angiotensin system. However, recent results, most notably those arising from the ACE-knock-out mice, indicate a much broader spectrum of physiological roles of this enzyme. ACE-/-mice not only have lower blood pressure, but they also have abnormalities in kidney structure and function, and the male mice have low fertility. In this application, it is hypothesized that each physiological function of ACE is the result of the expression of a specific isozyme in a specific tissue. To test this hypothesis, ACE expression will be restored in the ACE-/-mice, one tissue at a time, and the resultant effects on the physiological processes will be measured. A germ-cell specific promoter will be used to express transgenic ACET exclusively in the germ cells of mice. Similarly, a vascular endothelial cell-specific promoter and a kidney epithelial cell-specific promoter will be used to express transgenic ACEP exclusively in those tissues. These promoters will also be used to express the two isozymes of ACE in non-cognate tissues of transgenic mice. These transgenic mice will be cross-bred with ACE-/-mice for generating different strains of transgene +/+, ACE-/- mice each of which will express one isozyme of ACE in only one given tissue. Analyses of blood pressure, male fertility, and the structures and functions of kidneys of these mice will reveal the physiological consequences of tissue-specific and isozyme-specific expression of ACE. These studies may also lead to the identification of additional physiological substrates of this important enzyme.